Since the advent of the telephone system, copper wire was traditionally provided as the link in a "local loop" between a telephone subscriber and a local exchange. In spite of the 600 million telephone lines currently in use, four-fifths of the global population have never used a phone. This situation is rapidly changing as countries realize the relationship between a solid communications infrastructure and national prosperity. Countries with developing economies are faced with delivering cost-effective plain old telephone service (POTS) to hundreds of millions of impatient future subscribers. Installing twisted-pair bundles throughout the countryside or through crowded metropolitan areas is a slow, often uneconomic proposition. In some countries, wire laid between the customer premise and the local exchange in daylight hours is ripped up at night, to be sold for the value of the copper. Installing a twisted-pair-based public infrastructure requires immense amounts of time and money.
Wireless local loop (WLL) technology, used to complete the "last mile" of the subscriber loop, appears assured of being the technology solution of choice for the exploding worldwide telecommunications market. However, while WLL systems avoid the costs and delays associated with laying copper cable as an infrastructure or bypass solution, the convenience of a wireless solution presents new technological challenges. Key among them is achieving wireline voice quality in a wireless solution.
For equivalent performance, WLL systems require more elaborate transhybrid balance networks than traditional wired telephone systems. Due to the longer time delays in the speech path and the unpredictability of the terminating impedance, it is almost impossible to choose a single compromise balance network that will consistently deliver ideal transhybrid performance in a WLL system.
In a nonmobile wireless local loop system, the wireless basestation typically connects to the local exchange via a standard interface. The customer premise equipment consists of a box on the side of a house, which contains the radio unit and synthesizes the POTS interface. The customer terminal equipment (telephone, fax etc.) then plugs into the house side box in the same manner as with a wireline system.
The primary differences between a WLL system and a wired system are that the WLL system typically incorporates some type of speech compander, such as adaptive differential pulse code modulation (ADPCM), and the fmal wired loop from the box at the house to the terminal equipment is very short. Speech companders are generally employed to improve speech quality. As a consequence of using a wireless system, round-trip delays of up to 40 ms can occur with the dominant source of delay being a baseband protocol processor.
A potential source of echo is caused by a mismatch in impedances between the customer's terminal equipment and the balance network residing in the POTS interface. In short loops, such as those in a WLL network, the 2-wire impedance is dominated by the terminal equipment only, since the line is so short its impedance is insignificant. By contrast, in a traditional wireline system, the line can be very long, in which case the characteristic impedance of the line dominates, and the impedance of the terminal equipment becomes irrelevant. For this reason, it is usually possible to achieve acceptable performance by using a fixed but compromised balance network in a wired system.
Transhybrid balance is a name given to the degree of echo cancellation, for example, in a WLL system. Transhybrid balance is the ratio of the reflected signal to the transmitted signal (as viewed from the 4-wire side). In a WLL system, it is desirable to perform the transhybrid balance function near the customer's terminal equipment where the echo delay is a minimum. An attempt to perform this function at the wireless basestation or at the local exchange would require a more complicated network because of the increased delays the signal encounters while passing through the companders in the basestation and the box at the house. Due to the complicated nature of automatic transhybrid balance, it is best handled by a digital signal processor (DSP). If the coded function is DSP-based, the balance network may be implemented in the programmable digital domain.
The general implementation of automatic balance is handled in the following manner. An adaptive balance filter performs an estimate of an echo-path impulse response and dynamically adjusts a set of digital-filter coefficients to create an echo replica. This echo replica is inverted and summed into the transmit path to cancel the echo component in the transmit signal. The adaptation process using the echo residual signal to adjust the echo replica is repeated until the best echo cancellation is achieved. The net effect is that the trans-hybrid loss is kept to a minimum regardless of changing line or subscriber loop conditions.
A problem with many conventional automatic balance systems is that they employ a large number of finite impulse response (FIR) filter taps (e.g., 32 taps) in order to properly replicate the echo signal. The cost of an FIR filter is a function of the number of taps and thus FIR filters for many automatic balance systems tend to be rather expensive. In some automatic balance systems the FIR filter has associated therewith an adaptive infinite impulse response (IIR) filter. The adaptive IIR filter serves to reduce the number of taps required in the FIR filter since the IIR filter can generate an echo replica for a substantial portion of the echo signal provided that the echo signal is monotonically decaying. However, conventional automatic balance systems which employ an adaptive IIR filter are still very expensive and complicated because of the adaptive aspect of the IIR filter. In other words, the poles/zeros employed in the adaptive IIR filters dynamically change in order to provide an optimal long-tail echo replica. In order to implement such an adaptive IIR filter a complicated and expensive circuitry is generally required.
In view of the above, there is a need in the art for a simple and economical automatic balance system for canceling echo effects in WLL systems.